Guest-responsive coherence time of radical qubits in a metal-organic framework.

Metal-organic frameworks (MOFs) integrated with molecular qubits are promising for quantum sensing. In this study, a new UiO-type MOF with a 5,12-diazatetracene (DAT)-containing ligand is synthesized, and the radicals generated in the MOF exhibit high stability and a relatively long coherence time (T2) responsive to the introduction of various guest molecules.

Orbital hybridization of donor and acceptor to enhance the conductivity of mixed-stack complexes.

Mixed-stack complexes which comprise columns of alternating donors and acceptors are organic conductors with typically poor electrical conductivity because they are either in a neutral or highly ionic state. This indicates that conductive carriers are insufficient or are mainly localized. In this study, mixed-stack complexes that uniquely exist at the neutral-ionic boundary were synthesized by combining donors (bis(3,4-ethylenedichalcogenothiophene)) and acceptors (fluorinated tetracyanoquinodimethanes) with similar energy levels and orbital symmetry between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Surprisingly, the orbitals were highly hybridized in the single-crystal complexes, enhancing the room-temperature conductivity (10-4-0.1 S cm-1) of mixed-stack complexes. Specifically, the maximum conductivity was the highest reported for single-crystal mixed-stack complexes under ambient pressures. The unique electronic structures at the neutral-ionic boundary exhibited structural perturbations between their electron-itinerant and localized states, causing abrupt temperature-dependent changes in their electrical, optical, dielectric, and magnetic properties.

Radical qubits photo-generated in acene-based metal-organic frameworks.

A series of metal-organic frameworks (MOFs) assembled with diazatetracene (DAT)-based linkers were synthesized and characterized. Despite different chromophore orientations and spacings, photoinduced persistent radicals were generated in all the MOFs, and their spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) were found to be relatively long even at room temperature. The generality of long T1 and T2 values of photogenerated radicals in the chromophore-assembled MOFs provides a new platform towards quantum sensing applications.

Spin-Polarized Radicals with Extremely Long Spin-Lattice Relaxation Time at Room Temperature in a Metal-Organic Framework.

The generation of spin polarization is key in quantum information science and dynamic nuclear polarization. Polarized electron spins with long spin-lattice relaxation times (T1) at room temperature are important for these applications but have been difficult to achieve. We report the realization of spin-polarized radicals with extremely long T1 at room temperature in a metal-organic framework (MOF) in which azaacene chromophores are densely integrated. Persistent radicals are generated in the MOF by charge separation after photoexcitation. Spin polarization of a triplet generated by photoexcitation is successfully transferred to the persistent radicals. Pulse electron spin resonance measurements reveal that the T1 of the polarized radical in the MOF is as long as 214 µs with a relatively long spin-spin relaxation time T2 of the radicals of up to 0.98 µs at room temperature. The achievement of extremely long spin polarization in MOFs with nanopores accessible to guest molecules will be an important cornerstone for future highly sensitive quantum sensing and efficient dynamic nuclear polarization.

Clarifying the Electronic Structure of Anion-Templated Silver Nanoclusters by Optical Absorption Spectroscopy and Theoretical Calculation.

Electronic structures of anion-templated silver nanoclusters (Ag NCs) are not well understood compared to conventional, template-free Ag NCs. In this study, we synthesized three new anion-templated Ag NCs, namely [S@Ag17(S-4CBM)15(PPh3)5]0, [S@Ag18(S-4CBM)16(PPh3)8]0, and [Cl@Ag18(S-4CBM)16(PPh3)8][PPh4], where S-4CBM = 4-chlorobenzene methanethiolate, and single-crystal X-ray crystallography revealed that they have S@Ag6, S@Ag10, and Cl@Ag10 cores, respectively. Investigation of their electronic structures by optical spectroscopy and theoretical calculations elucidated the following unique features: (1) their electronic structures are different from those of template-free Ag NCs described by the superatomic concept; (2) optical absorption in the range of 550-400 nm for S2--templated Ag NCs is attributed to the charge transitions from S2--templated Ag-cage orbitals to the s-shaped orbital in the S2- moiety; (3) the Cl--templated Ag NCs can be viewed as [Cl@Ag18(S-4CBM)16(PPh3)8]0[PPh4]0 rather than the ion pair [Cl@Ag18(S-4CBM)16(PPh3)8]-[PPh4]+; and (4) singlet-coupled singly occupied orbitals are involved in the optical absorption of the Cl--templated Ag NC.

Metallic State of a Mixed-Sequence Oligomer Salt That Models Doped PEDOT Family.

Modern organic conductors are typically low-molecular-weight or polymer-based materials. Low-molecular-weight materials can be characterized using crystallographic information, allowing structure-conductivity relationships to be established and conduction mechanisms to be understood. However, controlling their conductive properties through molecular structural modulation is often challenging because of their relatively narrow conjugate areas. In contrast, polymer-based materials have highly π-conjugated structures with wide molecular-weight distributions, and their structural inhomogeneity makes characterizing their structures difficult. Thus, we focused on the less-explored intermediate, i.e., single-molecular-weight oligomers that model doped poly(3,4-ethylenedioxythiophene) (PEDOT). The dimer and trimer models provided clear structures; however, the short oligomers led to much lower conductivities (<10-3 S cm-1) than that of doped PEDOT. Herein, we elongated the oligomer to a tetramer through geometrical tuning based on a mixed sequence. The "P-S-S-P" sequence (S: 3,4-ethylenedithiothiophene; P: 3,4-(2',2'-dimethypropylenedioxy)thiophene) with twisted S-S enhanced the solubility and chemical stability. The subsequent oxidation process planarized the oligomer and expanded the conjugate area. Interestingly, the sequence involving sterically bulky outer P units allowed the doped oligomer to form a pitched π-stack in the single-crystal form. This enabled the inclusion of excess counter anions, which modulated the band filling. The combined effects of conjugate area expansion and band-filling modulation significantly increased the room-temperature conductivity to 36 S cm-1. This is the highest value reported for a single-crystalline oligomer conductor. Furthermore, a metallic state was observed above room temperature in a single-crystalline oligoEDOT for the first time. This unique mixed-sequence strategy for oligomer-based conductors enabled the precise control of conductive properties.

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]- (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy.

Au38(PET)24 (PET = SC2H4Ph) is known to have a bi-icosahedral Au23 core consisting of two Au13 icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au23 core is similar to that in the F2 molecule. The SVB model predicted that the electron configuration of the Au23 core with 14 valence electrons is expressed as (1Σ)2(1Σ*)2(1Π)4(2Σ)2(1Π*)4 where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au13 units. Therefore, the bi-icosahedral Au23 can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M1Au37(PET)24]- (M = Pd and Pt), which are isoelectronic and isostructural with Au38(PET)24. To this end, the neutral precursors [M1Au37(PET)24]0 were first synthesized by fusion reactions between hydride-doped clusters [HAu9(PPh3)8]2+ and [M1Au24(PET)18]-. The formation of bi-icosahedral M1Au22 cores with open electronic structure in [M1Au37(PET)24]0 was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M1Au37(PET)24]- were obtained by reducing [M1Au37(PET)24]0 with NaBH4, and isoelectronicity with [Au38(PET)24]0 was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M1Au37(PET)24]- observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

Pressure-Induced Superconductivity of the Quasi-One-Dimensional Organic Conductor (TMTTF)2TaF6.

We investigated the superconductivity of (TMTTF)2TaF6 (TMTTF: tetramethyl-tetrathiafulvalene) by conducting resistivity measurements under high pressure up to 8 GPa. A cubic anvil cell (CAC) pressure generator, which can produce hydrostatic high-pressure, was used for this study. Since the generalized temperature-pressure (T-P) diagram of (TMTCF)2X (C = Se, S, X: monovalent anion) based on (TMTTF)2PF6 (TCO = 70 K and spin-Peierls: SP, TSP = 15 K) was proposed by Jérome, exploring superconductivity states using high-pressure measurement beyond 4 GPa has been required to confirm the universality of the electron-correlation variation under pressure in (TMTTF)2X (TMTTF)2TaF6, which has the largest octahedral-symmetry counter anion TaF6 in the (TMTTF)2X series, possesses the highest charge-ordering (CO) transition temperature (TCO = 175 K) in (TMTTF)2X and demonstrates an anti-ferromagnetic transition (TAF = 9 K) at ambient pressure. A superconducting state in (TMTTF)2TaF6 emerged after a metal-insulator transition was suppressed with increasing external pressure. We discovered a superconducting state in 5 ≤ P ≤ 6 GPa from Tc = 2.1 K to 2.8 K, whose pressure range is one-third narrower than that of X = SbF6 (5.4 ≤ P ≤ 9 GPa). In addition, when the pressures with maximum SC temperatures are compared between the PF6 and the TaF6 salts, we found that (TMTTF)2TaF6 has a 0.75 GPa on the negative pressure side in the T-P phase diagram of (TMTTF)2PF6.

Conformational ensemble of a multidomain protein explored by Gd3+ electron paramagnetic resonance.

Despite their importance in function, the conformational state of proteins and its changes are often poorly understood, mainly because of the lack of an efficient tool. MurD, a 47-kDa protein enzyme responsible for peptidoglycan biosynthesis, is one of those proteins whose conformational states and changes during their catalytic cycle are not well understood. Although it has been considered that MurD takes a single conformational state in solution as shown by a crystal structure, the solution nuclear magnetic resonance (NMR) study suggested the existence of multiple conformational state of apo MurD in solution. However, the conformational distribution has not been evaluated. In this work, we investigate the conformational states of MurD by the use of electron paramagnetic resonance (EPR), especially intergadolinium distance measurement using double electron-electron resonance (DEER) measurement. The gadolinium ions are fixed on specific positions on MurD via a rigid double-arm paramagnetic lanthanide tag that has been originally developed for paramagnetic NMR. The combined use of NMR and EPR enables accurate interpretation of the DEER distance information to the structural information of MurD. The DEER distance measurement for apo MurD shows a broad distance distribution, whereas the presence of the inhibitor narrows the distance distribution. The results suggest that MurD exists in a wide variety of conformational states in the absence of ligands, whereas binding of the inhibitor eliminates variation in conformational states. The multiple conformational states of MurD were previously implied by NMR experiments, but our DEER data provided structural characterization of the conformational variety of MurD.

Tetramethyltetrathiafulvalene [(NbOF4)-]∞: One-Dimensional Charge Transfer Salt with an Infinite Anion Chain.

An unusual valence one-dimensional (1D) molecular charge transfer salt (TMTTF)(NbOF4) [TMTTF = tetramethyltetrathiafulvalene] with infinite anion chains was prepared. To understand the crystal structure and electronic states of the (TMTTF)(NbOF4) salt, we performed synchrotron X-ray diffraction, electron spin resonance, and static magnetization measurements. There is only one independent TMTTF molecule in the unit cell of (TMTTF)(NbOF4). The TMTTF1+ cation radicals stack to form 1D columns. The effective charge of the TMTTF molecule in the crystal was estimated to be +1. The electric charge of TMTTF donors is compensated by the infinite anion chains [(NbOF4)-]∞. The magnetic susceptibility of (TMTTF)(NbOF4) is 4 × 10-4 emu/mol at room temperature and shows weak temperature dependence above 60 K. However, some deviation appears below 60 K. The temperature dependence of the spin susceptibility shows a noticeable enhancement below 60 K. Below 5 K, the magnetization curve as a function of the magnetic field deviates from the straight line and shows a saturation tendency. The experimental results can be reproduced well with the S = 2 spin system at 2 K. The detailed analysis of the crystal structure and anomalous low-temperature magnetic state magnetic properties of (TMTTF)(NbOF4) are discussed.

Controlled Dimerization and Bonding Scheme of Icosahedral M@Au12 (M=Pd, Pt) Superatoms.

Targeted syntheses of MM'Au36 (PET)24 (M, M'=Pd, Pt; PET=SC2 H4 Ph) were achieved by hydride-mediated fusion reactions between [MAu8 (PPh3 )8 ]2+ and [M'Au24 (PET)18 ]- . Single-crystal X-ray diffraction analysis indicated that the products have bi-icosahedral MM'Au21 cores composed of M@Au12 and M'@Au12 superatoms. Although the MM'Au21 superatomic molecules correspond to O2 in terms of the number of valence electrons (12 e), the distances between the icosahedrons were larger than that in the bi-icosahedral Au23 core of Au38 (PET)24 corresponding to F2 and the spin state was singlet. These counterintuitive results were explained by a "bent bonding model" based on tilted (non-orthogonal) bonding interaction between the 1P superatomic orbitals of M@Au12 and M'@Au12 superatoms.

Understanding Doping Effects on Electronic Structures of Gold Superatoms: A Case Study of Diphosphine-Protected M@Au12 (M = Au, Pt, Ir).

Dopants into ligand-protected Au superatoms have been hitherto limited to group X-XII elements (Pt, Pd, Ag, Cu, Hg, and Cd). To expand the scope of the dopants to the group IX elements, we synthesized unprecedented [IrAu12(dppe)5Cl2]+ [IrAu12; dppe = 1,2-bis(diphenylphosphino)ethane] and [PtAu12(dppe)5Cl2]2+ (PtAu12) and compared their electronic structures with that of [Au13(dppe)5Cl2]3+ (Au13). Single-crystal X-ray diffractometry, 31P{1H} NMR, and Ir L3-edge extended X-ray absorption fine structure analysis of IrAu12 revealed that the single Ir atom is located at the center of the icosahedral IrAu12 core. Electrochemical analysis demonstrated that the energy levels of the highest occupied molecular orbitals are upshifted in the order of Au13 < PtAu12 < IrAu12. This trend was qualitatively explained in such a manner that the jellium core potential at the central position becomes shallower by replacing Au+ with Pt0 and further with Ir-. IrAu12 underwent reversible redox reactions between the charge states of 1+ and 2+. The gradual increase of the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital in the order of Au13 < PtAu12 < IrAu12 was observed by electrochemical measurement and optical spectroscopy. This study provides a simple guiding principle to tune the electronic structures of heterometal-doped superatoms.

Stoichiometric Formation of Open-Shell [PtAu24(SC2H4Ph)18]- via Spontaneous Electron Proportionation between [PtAu24(SC2H4Ph)18]2- and [PtAu24(SC2H4Ph)18]0.

[PtAu24(SC2H4Ph)18]0 ([PtAu24]0) was fully and selectively converted to [PtAu24]-, having an open electronic structure with seven valence electrons, upon the addition of an equiamount of NaBH4. Stoichiometric production of [PtAu24]- by the reaction between an equal amount of [PtAu24]0 and [PtAu24]2- revealed that the above reaction proceeds via the spontaneous electron transfer (ET) from [PtAu24]2- nascently reduced by H--mediated reduction to [PtAu24]0 remaining in the solution. Theoretical calculation suggested that the driving force of this novel ET reaction was the larger adiabatic electron affinity of [PtAu24]0 compared to that of [PtAu24]-, partly associated with reduction-induced relief of the Jahn-Teller strain. We propose that ET proceeds via the dimer complex of [PtAu24]2- and [PtAu24]0 formed through the aurophilic interaction between Au(I) sites in the surface layer.

A Peptoid with Extended Shape in Water.

The term "peptoids" was introduced decades ago to describe peptide analogues that exhibit better physicochemical and pharmacokinetic properties than peptides. Oligo(N-substituted glycine) (oligo-NSG) was previously proposed as a peptoid due to its high proteolytic resistance and membrane permeability. However, oligo-NSG is conformationally flexible, and ensuring a defined shape in water is difficult. This conformational flexibility severely limits the biological application of oligo-NSG. Here, we propose oligo(N-substituted alanine) (oligo-NSA) as a peptoid that forms a defined shape in water. The synthetic method established in this study enabled the first isolation and conformational study of optically pure oligo-NSA. Computational simulations, crystallographic studies, and spectroscopic analysis demonstrated the well-defined extended shape of oligo-NSA realized by backbone steric effects. This new class of peptoid achieves the constrained conformation without any assistance of N-substituents and serves as a scaffold for displaying functional groups in well-defined three-dimensional space in water, which leads to effective biomolecular recognition.

Bacterial Fermentation of Water-Soluble Cellulose Acetate Raises Large-Bowel Acetate and Propionate and Decreases Plasma Cholesterol Concentrations in Rats.

We hypothesized that water-soluble cellulose acetate (WSCA) could be useful tool for the delivery of short-chain fatty acids to the large intestine. Rats were fed a control diet or a diet containing graded levels of WSCA for up to 21 days. Consuming WSCA dose-dependently increased large-bowel acetate and propionate concentrations through the bacterial fermentation. When WSCA was used as substrate, acetyl esterase activity in the cecal bacteria was detected solely in rats fed WSCA, in which the activity increased over time accompanied by an increased number of Bacteroides xylanisolvens. Consuming WSCA at a 4% level increased the goblet cell numbers and mucin contents in the cecum and lowered plasma cholesterol concentrations, which tended to correlate with the portal plasma concentrations of propionate. The results suggest that bacterial fermentation of WSCA is characterized by the greater production of acetate and propionate, which may contribute to the physiologic alterations.

HGF induces the serine­phosphorylation and cell surface translocation of ROMK (Kir 1.1) channels in rat kidney cells.

Extracellular potassium homeostasis is dependent on the activity of potassium channels, which are expressed on the apical membrane of epithelial tubular cells. The renal outer medullary potassium channel (ROMK) is considered to be the major route for potassium transport into the tubule lumen. Hepatocyte growth factor (HGF) exerts multiple biological activities and is important for maintaining renal homeostasis. It is also anti­apoptotic and mitogenic for protection and recovery from ARF. Whether HGF regulates the ion channel activities remains to be elucidated, therefore, the present study aimed to investigate the modulation of HGF on the expression of ROMK in cultured renal tubular cells. NRK­52E cells were treated with recombinant HGF, however, no alterations in the total expression of ROMK were observed by western blot analysis. In examining the serine 44 phosphorylation of ROMK in NRK­52E cells, the present study observed that HGF enhanced the serine 44 phosphorylation of ROMK. In addition, to investigate whether HGF­Met signaling induces the movement of ROMK to the cell surface in NRK­52E cells, the protein constituents of cells were separated into plasma membrane and cytoplasm. Using immunofluorescence assay, the expression of ROMK on the plasma membrane was increased in the HGF­treated NRK­52E cells, which suggested that ROMK was translocated to the plasma membrane following the HGF­induced phosphorylation of serine 44. Therefore, HGF may be important in potassium excretion and perform antihyperkalemic effects through the translocation of potassium channels.

Successive Dimensional Transition in (TMTTF)_{2}PF_{6} Revealed by Synchrotron X-ray Diffraction.

A quasi-one-dimensional organic charge-transfer salt (TMTTF)_{2}PF_{6} undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a "structureless transition," and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer δ_{CO}=0.20e. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

Two-dimensional sp2 carbon-conjugated covalent organic frameworks.

We synthesized a two-dimensional (2D) crystalline covalent organic framework (sp2c-COF) that was designed to be fully π-conjugated and constructed from all sp2 carbons by C=C condensation reactions of tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile. The C=C linkages topologically connect pyrene knots at regular intervals into a 2D lattice with π conjugations extended along both x and y directions and develop an eclipsed layer framework rather than the more conventionally obtained disordered structures. The sp2c-COF is a semiconductor with a discrete band gap of 1.9 electron volts and can be chemically oxidized to enhance conductivity by 12 orders of magnitude. The generated radicals are confined on the pyrene knots, enabling the formation of a paramagnetic carbon structure with high spin density. The sp2 carbon framework induces ferromagnetic phase transition to develop spin-spin coherence and align spins unidirectionally across the material.

Tryptophan-2,3-Dioxygenase (TDO) deficiency is associated with subclinical neuroprotection in a mouse model of multiple sclerosis.

The catabolism of tryptophan to immunosuppressive and neuroactive kynurenines is a key metabolic pathway regulating immune responses and neurotoxicity. The rate-limiting step is controlled by indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO). IDO is expressed in antigen presenting cells during immune reactions, hepatic TDO regulates blood homeostasis of tryptophan and neuronal TDO influences neurogenesis. While the role of IDO has been described in multiple immunological settings, little is known about TDO's effects on the immune system. TDO-deficiency is neuroprotective in C. elegans and Drosophila by increasing tryptophan and specific kynurenines. Here we have determined the role of TDO in autoimmunity and neurodegeneration in experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. We created reporter-TDO mice for in vivo imaging to show that hepatic but not CNS TDO expression is activated during EAE. TDO deficiency did not influence myelin-specific T cells, leukocyte infiltration into the CNS, demyelination and disease activity. TDO-deficiency protected from neuronal loss in the spinal cord but not in the optic nerves. While this protection did not translate to an improved overt clinical outcome, our data suggest that spatially distinct neuroprotection is conserved in mammals and support TDO as a potential target for treatment of diseases associated with neurodegeneration.

Negative Impact of Hypoxia on Tryptophan 2,3-Dioxygenase Function.

Tryptophan is an essential amino acid for hosts and pathogens. The liver enzyme tryptophan 2,3-dioxygenase (TDO) provokes, by its ability to degrade tryptophan to N-formylkynurenine, the precursor of the immune-relevant kynurenines, direct and indirect antimicrobial and immunoregulatory states. Up to now these TDO-mediated broad-spectrum effector functions have never been observed under hypoxia in vitro, although physiologic oxygen concentrations in liver tissue are low, especially in case of infection. Here we analysed recombinant expressed human TDO and ex vivo murine TDO functions under different oxygen conditions and show that TDO-induced restrictions of clinically relevant pathogens (bacteria, parasites) and of T cell proliferation are abrogated under hypoxic conditions. We pinpointed the loss of TDO efficiency to the reduction of TDO activity, since cell survival and TDO protein levels were unaffected. In conclusion, the potent antimicrobial as well as immunoregulatory effects of TDO were substantially impaired under hypoxic conditions that pathophysiologically occur in vivo. This might be detrimental for the appropriate host immune response towards relevant pathogens.